This works, but for very large formulas the double delimiters are not sufficiently tight, and the negative kerning should be larger.

On the other hand, I've found some code that allows to define double delimiters
and doesn't seem to suffer from the same problem. The following is from a
newsgroup posting, it defines \lleft and \rright so that e.g.
\lleft\langle results in <<.

Works almost, except for the second vertical in e.g. \LBraket{\rho| \hat{\hat{A}} | \rho}, the way \Braket does. Would there be a way to incorporate that as well?
–
Michael GoerzOct 27 '11 at 20:26

Well, that indeed does two bars, but unlike the first version, it doesn't seem to scale the negative spacing between the two delimiters. Actually, if someone can pick it apart, I'd love to be able to understand how the original \lleft macro works... what's the trick there?
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Michael GoerzOct 28 '11 at 11:39